Referring to FIG. 1, a conventional ring laser gyro will be outlined. A gyro block 11 made of glass has a triangular path 12 formed therewithin and mirrors 13, 14, 15 disposed on the apexes of the triangle formed by the path 12. The path 12 has a laser medium enclosed therewithin and anodes 16, 17 and cathode 18 disposed on the sides of the path 12. When a high voltage is applied across the anodes 16, 17 and cathode 18, the laser medium is excited and clockwise and counterclockwise laser beams oscillate. The clockwise and counterclockwise laser beams reflect off the mirrors 13, 14, 15 and propagate through the path 12 in a triangular ring shape. The optical paths of the laser beams will be referred to hereinafter as ring-shaped optical paths.
If an angular velocity is input into the gyro block 11 in this state, a difference arises between the optical path lengths of the laser beams that propagate in mutually opposite directions (clockwise and counterclockwise). This difference between the optical path lengths causes an oscillation frequency difference between the laser beams. Superposing the clockwise and counterclockwise laser beams on each other produces an interference pattern from which an angular velocity can be detected.
To this end, the laser beams are taken out through the mirror 13 which is semitransparent and serves as the readout mirror. In FIG. 1, reference numeral 21 denotes a photosensor for detecting the interference pattern (interference light), reference numeral 22 denotes a prism for refracting the optical path of one of the laser beams to form the interference pattern with the other laser beam. Reference numeral 23 denotes a laser beam intensity detector for measuring the intensity of one of the laser beams (i.e., beam intensity), which has been taken out through the mirror 13, in order to control the optical path lengths of the ring-shaped optical paths. Note that in FIG. 1 any mirror transducers are not shown which controls the optical path lengths so as to keep constant the beam intensities measured by the laser beam intensity detector 23.
Although the photosensor 21, prism 22, and laser beam intensity detector 23 are schematically shown as components separate from the gyro block 11, these components are attached to the gyro block 11 or a gyro case to which the gyro block 11 is secured.
The gyro block 11 has an opening 19 defined at the center thereof and a dither mechanism 31 attached to the opening 19 to apply vibrations in the laser beam propagating directions, i.e., vibrations about the axial center of the ring-shaped optical path, to the gyro block 11 to reduce a lock-in phenomenon.
As shown in FIG. 2, the dither mechanism 31 includes a cylindrical movable portion 32, three arm-like deforming portions 33 radially extending from its axial center to the movable portion 32, and a fixed portion 34 connected to the deforming portions 33 at the axial center and having three island-like fitting portions 34a protruding to the spaces delimited by the deforming portions 33, the deforming portions 33 having piezoelectric elements 35 bonded to both side surfaces thereof.
Out of the three pairs of (i.e., six) piezoelectric elements 35 attached to the three deforming portions 33, two pairs are used to drive the dither mechanism 31 and the remaining one pair is used to detect dithers. The fitting portions 34a have counterbored holes 34b formed therein to receive screws for securing the ring laser gyro equipped with the dither mechanism 31 to the gyro case, for example.
A dither control unit 43 drives and controls the dither mechanism 31 so as to apply vibrations to the gyro block 11 to keep constant the amplitude of the electric signal (referred to hereinafter as the dither pick-off signal) obtained through the piezoelectric elements for dither detection. The dither pick-off signal is also input to a bias signal eliminating unit 42.
Information (such as the direction and speed of movement) of the interference pattern detected by the photosensor 21 is input to a signal processing unit 41 and converted into angular velocity information. The angular velocity information output by the signal processing unit 41 is input to the bias signal eliminating unit 42.
The bias signal eliminating unit 42 uses the dither pick-off signal to eliminate from the angular velocity information a vibrational angular velocity signal corresponding to the dither vibration applied in the laser beam propagating direction and outputs the resultant signal as an angular velocity signal. This angular velocity signal represents the angular velocity of a moving apparatus equipped with the ring laser gyro.
This type of ring laser gyro is disclosed in Japanese Patent Application Laid Open Nos. 2010-127686, 2008-309704, and 2007-93551.